Data loading

The data is read from ISF files, using the isfreader library. It is installed bellow automatically if not already installed.

Sampling frequency in kHz

Spectral analysis of signals

The mixer signals is a base sine wave (the envelope of the mixing) at a frequency close to 500 kHz. The reference saw-tooth frequency sweeping wave has the same base frequency, but with a trail of harmonics forming the sharp saw-tooth shape.

Base band signal analysis

The difference between the phase of the reference and mixer signal is proportional to the line-integrated electron density. Therefore, it is necessary to obtain the "phase" of both signals.

The instantanous phase and amplitude of the base signal can be inferred only for the base bad, i.e. by removing higher and lower frequencies (i.e. extracting the base sine wave from the reference saw-tooth signal)

The instantanous phase and amplitude of this base band signal can be obtained from the analytic signal using the Hilbert transform. For convenience of processing and visualization, the phase is unwrapped ($\pi$ discontinuities are shifted) and it's time derivative, the instantaneous frequency, is calucated as well.

For convenience of visualization it is convenient to subtract the phase evolution of the base nad signal from the phases (which is just a linear trend). This base trend vanishes in the final phase difference calculation anyways as long as the same trend is removed. Therefore, the referense saw-tooth signals is used for the trend estimation.

The signal boundaries may be distorted by the filter, so they are not used.

The phase difference between the interferometer signal with respect to the reference signal is proportional to the line-averaged density. An arbitrary initial phase difference is subtracted.